1. Field of the Invention
The present invention generally relates to a frequency modulating circuit and, more particularly, to a circuit adapted for use in a video tape recorder (VTR) which magnetically records video signals.
2. Description of the Prior Art
In the conventional video tape recorder known heretofore, a video signal or a luminance signal extracted therefrom is magnetically recorded through frequency modulation.
As shown in FIG. 1, a video signal SV is fed to a synchronizing signal separating circuit 2, which then produces a gate signal SP1 rising at a predetermined timing.
In response to the gate pulse SP1, a clamp circuit 3 clamps the video signal SV in such a manner that a sync-tip level thereof becomes equal to a predetermined signal level.
A preemphasis circuit 4 emphasizes a high-range component of the video signal SV1 outputted from the clamp circuit 3, so as to prevent deterioration of the signal-to-noise ratio of the video signal in a recording reproduction mode. And a video signal SV2 produced with overshoot and undershoot as a result of such emphasis is outputted via a white clip circuit 5 and a dark clip circuit 6 to a modulating circuit 7.
The white clip circuit 5 and the dark clip circuit 6 serve to clip the video signal SV2 at predetermined white and dark signal levels respectively so that a frequency modulated signal SFV obtained via the modulating circuit 7 may not be overmodulated due to the overshoot and undershoot of the video signal SV2 beyond desired signal levels.
Thus the video signal SFV modulated at a predetermined frequency can be obtained via the modulating circuit 7 and then is outputted via an amplifying circuit to a magnetic head, whereby the video signal can be recorded on a magnetic tape.
However, in the video tape recorder of the type mentioned, the level of the reproduced signal is lowered with increase of the frequency magnetically recorded, so that the carrier frequency of the FM video signal SFV needs to be set at a low frequency close to the frequency band of the premodulation video signal SV2.
Therefore in the demodulating circuit, as shown in FIGS. 2 and 3, the FM video signal SFV modulated by a predetermined carrier frequency fm and obtained via the magnetic head is doubled to produce an FM video signal SFV2 modulated by a double carrier frequency 2fm. And from a sum signal of such video signal SFV2 and a demodulated video signal SV3, the latter signal SV3 alone is extracted via a low-pass filter circuit so that the FM video signal SFV is not mixed with the demodulated video signal SV3.
In the video tape recorder of the above type, however, there exists a problem that when the band of the video signal SV to be recorded is widened for the purpose of enhancing the reproduced picture quality, as shown in FIG. 4, the band of the demodulated video signal SV4 also becomes wider to eventually bring about a disadvantage that the high range of the video signal SV4 overlaps the low range of the video signal SFV2 modulated by the double carrier frequency 2fm.
Due to such mutual overlap of the bands, it becomes difficult to attain complete separation into the video signal SV4 and the frequency modulated video signal SFV2, and accordingly the sideband component of the video signal SVF2 is partially mixed into the video signal SV4 obtained via the low-pass filter circuit.
Consequently, in modulation of a video signal SV5 where an inverse 2T pulse P2 of a signal level varying inversely is superposed on a 2T pulse Pl whose half-amplitude level corresponds to a duration of 2T, as shown in FIG. 5, the inverse 2T pulse has a frequency of 5 MHz or so which is extremely close to the modulation carrier frequency. However, in the demodulated video signal SV4 shown in enlarged views of FIG. 6, there occur variations in the pulse durations T1, T2 and T3 of the inverse 2T pulse P2 (FIG. 6 (A), (B) and (C)) in accordance with the phase of the FM video signal SFV relative to the inverse 2T pulse P2, whereby the rise and fall instants of the inverse 2T pulse P2 are also varied. In case the frequency component of the video signal is considerably different from the modulation carrier frequency, many carrier waves are existent per pulse of the video signal so that, despite a slight phase deviation, the video signal can be reproduced with high fidelity by frequency modulation. However, in the above example where the video signal has an extremely narrow pulse duration such as the inverse 2T pulse, the number of the carrier waves per pulse is so small that even a slight phase deviation causes marked deterioration in the reproducibility of the original video signal.
If the rise and fall instants are varied in accordance with the phase of the FM video signal SFV in an exemplary case of a reproduced picture where a dark vertical line is displayed on a white background, it signifies that the horizontal display positions of the rise and fall portions of such dark vertical line are varied in accordance with the phase of the FM video signal SFV. Therefore, when the signal phase is varied per line of the displayed picture, the edge position of the vertical line is also varied in accordance with such phase variation.
Consequently, if the variation per line is periodically changed in synchronism with the display of each field, it causes distortion in the display with the dark vertical line becoming thicker or thinner. To the contrary, if the phase variation per line fails to be synchronous with the display of each field, the portion rendered thicker or thinner correspondingly to the synchronism error is shifted upward or downward to eventually induce a moire pattern on the display screen.
Practically the carrier frequency always has a slight drift in the modulating circuit 7, and occurrence of such moire pattern is unavoidable in the displayed picture if the band of the video signal SV4 is wide. Thus, there has been a problem heretofore that the deterioration caused in the reproduced picture quality by such moire pattern brings about a greater disadvantage as compared with the improvement attained in the reproduced picture quality by widening the band of the video signal SV.
For solving the problem mentioned, there may be contrived a method which sets low levels to limit the white and dark signals respectively in the white clip circuit 5 and the dark clip circuit 6 so as not to widen the sideband of the FM video signal SFV.
In this case, the sideband of the FM video signal SFV2 obtained by doubling the modulation frequency is also rendered narrower correspondingly, whereby the sideband component mixed into the demodulated video signal SV4 can be reduced.
Meanwhile, if the amplitude of the predetermined video signal SV2 is limited by lowering the respective signal levels of the white and dark clip circuits, there arises another problem that a waveform distortion is induced in the video signal obtained by deemphasizing the demodulated video signal SV4.
There may be contrived another method which increases the modulation carrier frequency fm to avert an overlap between he band of the doubled video signal SFV2 and the demodulated video signal SV4. But still some difficulties are existent in practical use since the signal-to-noise ratio in the high range of the reproduced signal is deteriorated with increase of the carrier frequency fm.